Artificial knee. Coloured computed tomography (CT) scan of an artificial (prosthetic) knee joint. The prostheses are implanted into the thigh bone (femur) and shinbone (tibia) with pegs and surgical cement. The artificial joint hinges in an identical way to a normal knee joint. Knee joints need replacing when the protective cartilage that surrounds them is lost. Cartilage can be lost due to injury or osteoarthritis.